This invention is directed to a process for the distillation of readily polymerizable vinyl aromatic monomers such as styrene, substituted styrene and vinyl benzenes, and more specifically to a process for the distillation of vinyl aromatic monomers wherein the degree of polymerization of the monomer is substantially reduced, under distillation conditions, by the presence of small but effective amounts of mixtures containing 2,6-dinitro-4-halophenols (Structure I). ##STR1## wherein X equals flourine, chlorine, bromine or iodine.
It is generally known that vinyl aromatic compounds such as monomeric styrene and the alkylated sytrenes such as methyl styrene, ethyl styrene, etc. are easily polymerized under distillation conditions and more particularly the rate of polymerization of the monomers increases substantially at increased temperatures. Many of the vinyl aromatic monomers available commercially, generally contain a variety of impurities and therefore these monomers need be purified, i.e. by distillation, to obtain the substantially pure monomer, i.e. styrene. As indicated, the purification and separation of these impurities from the monomers are generally accomplished by distillation. Thus, in order to prevent polymerization of the monomers during the distillation purification process, various compounds are added to the distillation as polymerization inhibitors.
For example, sulfur has been employed as a polymerization inhibitor. However, sulfur while being a reasonable effective inhibitor, has many disadvantages in that the waste materials formed are highly contaminated and therefore are of little or no value. Other inhibitors include alkylated catechols, hydroquinone, and a number of the halogen-substituted cresols such as 2-chloro-6-nitro-paracresol, 2-bromo-6-nitro- paracresol and mixtures of these materials as specifically taught in U.S. Pat. No. 4,132,602. In addition, other inhibitors include 3-nitro-2,5-cresotic acid and 3-nitro-2,5-cresotaldehyde as taught in U.S. Pat. No. 4,132,601. Still further, other vinyl inhibitors include the halogenated toluenes such as 4-halo-3,5-dinitrotoluene as taught in U.S. Pat. No. 4,132,603 and the nitrocresols as taught in U.S. Pat. No. 4,086,147.
While most of these compounds are effective for inhibiting the polymerization of vinyl aromatic compounds, not all of the compounds have been successful as polymerization inhibitors under the stringent conditions of distillation. It was found that notwithstanding the use of these inhibitors, the distillation of crude styrene, for example, resulted in products containing substantial quantities of polymeric material which is difficult to separate from the monomer.
More important, a number of the vinyl aromatic inhibitors have high melting points, e.g. dinitro paracresol has a melting point of about 85.degree. C. and therefore is transported as a solid. Thus, before using dinitro paracresol, it must be dissolved in solvents, e.g. ethyl benzene which subsequently requires separation. This requires additional equipment and adds to the process time and increased cost. Moreover, it is necessary to add these inhibitors as solutions of ethyl benzene to the monomer, e.g. styrene, since dinitro paracresol can not be held at temperatures greater than 85.degree. C. for any period of time due to the potential danger of side reactions and because of the explosive nature of the compound.
In comparison, mixtures of 2,6-dinitro-4-halophenols and 2,6-dinitro-4-ethylphenol have lower melting points and therefore can be transported as a liquid and metered directly into the process requiring no separation or other equipment.
Accordingly, it is an object of this invention to provide an effective polymerization inhibitor for distillation of vinyl aromatic compounds particularly styrene, and vinyl benzenes at elevated temperatures, which are easy to use and less costly. It is another object of this invention to provide an effective polymerization inhibitor for styrene and vinyl benzenes which are liquid at ambient temperatures and can be added directly to the distillation process without any prior treatment or handling.
These and other objects of the invention will become apparent from a further and more detailed description of the invention as follows. Specifically, this invention relates to a process for distilling polymerizable vinyl aromatic monomers such as styrene, substituted styrenes, alkyl benzenes and the like by subjecting the vinyl aromatic monomers to distillation temperatures in the presence of small but effective amounts of mixtures of 2,6-dinitro-4-halophenols, e.g. 2,6-dinitro-4-chloro, 2,6-dinitro-4-bromo, or 2,6-dinitro-4-iodophenol, with 2,6-dinitro-4-ethylphenol. The dinitro-4-halophenols are present in amounts ranging from about 1.0% to 99% by weight of the mixture.
Generally, distillation is carried out in the presence of effective amounts of the polymerization inhibitor ranging from about 50 to 3,000 parts by weight of the inhibitor per million parts by weight of the vinyl aromatic monomer and preferrably in amounts ranging from about 100 to 1000 parts per million parts by weight of the vinyl monomer. The temperatures at which the distillation is generally carried out ranges from about 40.degree. C. to 160.degree. C. and depending on the particular monomer being distilled and the temperatures utilized, the amount of inhibitor added to the system will vary within the range stated.
With some monomers it may be desirable to use mixtures of the 2,6-dinitro-4-halophenols with 2,6-dinitro-4-ethylphenol, wherein the dinitro-4-halophenol is present in amounts ranging from about 1.0% to 99% by weight of the mixture. The mixture may be added to the distillation of the monomer in amounts ranging from about 100 to 1000 parts per million parts by weight of the vinyl aromatic monomer. As stated, 2,6-dinitro-4-ethylphenol has a melting point of about 36.degree. C. and can be transported in the liquid form, i.e. at temperatures ranging from about 38.degree. C. to 40.degree. C. and metered directly into the distillation. Mixtures of 2,6-dinitro-4-ethylphenol and 2,6-dinitro-4-halophenol are liquid at ambient temperatures and can be metered in the same manner as 2,6,-dinitro-4-ethylphenol.